1. Field of the Invention
The present invention relates to miniature fuel cells and, more specifically, to a device and a method for controlling the humidification of the electrolyte of a fuel cell.
2. Discussion of the Related Art
Hydrogen-oxygen fuel cells having their upper surface forming the cathode of the cell exposed to a hydrogen source are here considered. In such a cell, an electrolyte is sandwiched between the anode and the cathode. On the anode side, an oxidation reaction transforms the oxygen into H+ ions which cross the electrolyte. On the cathode side, the H+ ions having crossed the electrolyte react with the air oxygen to form water. The circulation of the H+ ions (and of complementarily-formed electrons) ensure the cell operation.
The electrolyte is generally formed of a polymer membrane, for example, made of Nafion (trademark of DuPont Corporation). The conductivity to H+ ions of such electrolytes is substantially constant if the humidity ratio of the electrolyte is within a range of values, but strongly decreases if the humidity ratio of the electrolyte falls below this range.
Once the fuel cell has started, the water generated at the cathode ensures that the humidity ratio of the cell remains sufficient. However, for example, when the fuel cell has not been used for a long time and has been kept in a dry and possibly hot atmosphere, the material forming the electrolyte may dry out. In this case, the fuel cell will not start or only weakly so.
FIG. 1 is a curve illustrating the starting problems of an air-breathing fuel cell having its electrolyte in a dried out state. At an initial time when the cell is supplied with hydrogen, it delivers a current density approximately equal to 0.02 A/cm−2. The current flow contributes to heating the cell and the water generated at the cathode evaporates quickly. Thus, the fuel cell electrolyte further dries out, which decreases the current density delivered by the cell down to a value smaller than 0.01 A/cm−2 ten minutes after starting. The current density in the fuel cell thus does not reach the desired nominal values (on the order of from 0.7 to 0.9 A/cm−2).
To solve this problem, it has been provided in prior art (see for example patent application WO 2006/012953 or U.S. Pat. No. 6,830,841) to control the humidification of the electrolyte of a fuel cell by adjusting the humidity ratio of the gases supplied to the cell. Such systems for modifying the humidity ratio of the gases supplied to the cell are relatively complex and are not adapted to simple fuel cells. Such systems are also not adapted to miniature fuel cells, especially fuel cells intended to power small electronic devices such as cell phones.
To maintain the humidity of a fuel cell, it has also been provided to cover the cell cathode surface with a cap which is closed during periods when the cell is not used (see US-A1-2007/218338, JP-A-2005/032517, US-A1-2007/228740). This solution is ineffective in the case where the cell is not used for long periods, since it dries out after some time, however well protected it may be.